''DataColumn'' ''diposable'

DataColumn

diposable

System.Data (system.data.dll)

class

The DataColumn represents a column schema within a
table. The DataTable stores the
DataColumn objects in its
DataTable.Columns collection. The
DataColumn object doesn't contain
data. Instead, it defines the properties of a column such as the .NET
data type (DataType ), whether it allows null
values (AllowDBNull ), whether it is read-only
(ReadOnly ), whether values in each row of the
column must be unique (Unique ), the column name
(ColumnName ), the default value when a new row is
created (DefaultValue ), the maximum length for
text fields (MaxLength ), and the position
relative to other columns in the table (Ordinal ).
If you retrieve data without schema information, some of these values
(such as MaxLength ) aren't
initialized. However, even if you do retrieve schema information,
you'll find that some information, such as
DefaultValue , is never retrieved from the data
source.

The DataColumn class provides additional
properties, such as ExtendedProperties , which is
a collection you can use to store miscellaneous information, and
ColumnMapping , which allows you to configure the
XML representation for each column in a DataTable
using a value from the MappingType enumeration.
Finally, the Expression property allows you to
define a calculated or aggregate column or one that is stored as an
expression or calculation. For example, you use a statement such as:

dt.Columns("Total").Expression = "Price * 1.15";

to define an expression for a column that shows the tax-adjusted
value from the Price column. It assumes
you've created and added the
Total column. For a complete example, as well as
information about the syntax used for expression functions,
operators, and table relations, refer to Chapter 8.